build error: unable to create a static build without TIFF

configuring qt with "-static -no-libtiff" will still try to build the tiff plugin, leading to unresolved references later on (same for other image formats).

trouble here is that configure has a CFG_TIFF and a CFG_LIBTIFF. CFG_TIFF is always set to "yes" in a static build, ignoring the -no-libtiff parameter. In my case, I want to disable TIFF altogether, and I don't see how (without hacking .qmake.cache manually).

Note: To set CFG_TIFF to "no" if -no-libtiff is specified is wrong, since in a static build, libtiff can be linked later on (when linking the application).